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The perfect atom sandwich requires an extra layer

August 4th, 2014 ›

By Anne Ju

Yuefeng Nie

The left figure demonstrates why the
first double layer of strontium oxide is missing when growing a
Ruddlesden-Popper oxide thin film. Titanium atoms (yellow) preferentially
bond with oxygen atoms (gray) and sit at the center of a complete octahedron,
making it energetically more favorable for titanium to switch positions with
the topmost strontium oxide layer (red). Because of this, the first double
layer of strontium oxide is always missing, and the extra layer rides the
surface. By depositing an extra strontium oxide layer first, the desired
first double layer is obtained.

Like
the perfect sandwich, a perfectly engineered thin film for electronics requires
not only the right ingredients, but also just the right thickness of each
ingredient in the desired order, down to individual layers of atoms.

The
team, led by thin-films expert Darrell
Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry in the
Department of Materials Science and Engineering, describes the trick of growing
perfect films of oxides called Ruddlesden-Poppers in Nature Communications Aug.
4.

These
oxides are widely studied for their electronically enticing properties, among
them superconductivity, magnetoresistance and ferromagnetism. Their layered
structure is like a double Big Mac with alternating double and single layers of
meat patties - strontium oxide - and bread - titanium oxide - in the case of
the Ruddlesden-Poppers studied.

"Our
dream is to control these materials with atomic precision," Schlom said. "We
think that controlling interfaces between Ruddlesden-Poppers will lead to
exotic and potentially useful, emergent properties."

Schlom's
lab makes novel thin films with molecular beam epitaxy, a deposition method
that controls the order in which atom-thick layers are assembled
layer-by-layer, which Schlom likens to precision spray-painting with atoms.

In
experiments designed by first author and postdoctoral associate Yuefeng Nie,
the researchers found a major difference between assembling atomically precise
Ruddlesden-Popper films and the conventional layer-by-layer "sandwich making"
of molecular beam epitaxy.

This
discovery began when co-author Lena Kourkoutis, then a graduate student and now
assistant professor of applied and engineering physics, noticed that sample
after sample of Ruddlesden-Popper films spray-painted by Schlom's lab were
missing a layer of strontium oxide.

"Imagine
laying down two meat patties on a bun, followed by a layer of bread, and
another two meat patties, only to find that the resulting sandwich consists of
just one meat patty below the layer of bread and three above it," Nie
explained. "This is the equivalent of what we found to occur with our layers of
atoms."

"After
a while we asked, what's going on?" said co-author David A. Muller, professor
of applied and engineering physics. "Where did that first layer go?"

It
turned out that following a double layer of strontium and oxygen, the next
layer of titanium atoms, instead of sitting on top as expected, seeps down
between the two strontium oxide layers. That meant the missing first layer of
strontium and oxygen ended up on the film's surface - a subtlety overlooked for
years.

"This
paper is about understanding that this flipping is going on," Schlom said. "The
final sandwich structure is not simply the order in which we lay down the
layers."

The
researchers also designed a modification to their crystal growth to make a
Ruddlesden-Popper film - this time truly perfect - by laying down an extra
layer of strontium oxide to start. Understanding this growth process has helped
them make atomically precise and sharp interfaces between Ruddleseden-Poppers,
which paves the way for exploring and harnessing their useful properties for
devices.

A
competing paper by Argonne National Laboratory researcher June Lee, a graduate
of Schlom's group, and published the same week in Nature Materials, arrived at
similar conclusions by using different methods.

The
Cornell paper, "Atomically Precise Interfaces From Non-Stoichiometric
Deposition," was also co-authored by postdoctoral associate Ye Zhu; graduate
students Che-Hui Lee, Julia Mundy, David Baek and Suk Hyun Sung; and Kyle Shen,
associate professor of physics and member of the Kavli Institute at Cornell for
Nanoscale Science. The work was supported by the Army Research Office, the
National Science Foundation and the Kavli Institute, of which Muller is
co-director.